The objective of the present invention is a method of producing CdTe solar cells with increased efficiency.
The distribution of thin-film solar cells may be accelerated further by increasing their electric efficiency in light conversion. Solar cells based on CdTe have proven particularly promising in this respect.
In the state of the art, the CdTe solar cell has the following structure: on a glass substrate, a transparent conducting oxide layer (TCO) is deposited as front contact. The TCO layer can include a high resistive buffer layer which helps to minimise the shunting effect in solar cell. On this, a layer of cadmium sulfide (CdS) and on top of that, a layer of cadmium telluride (CdTe) are deposited. Finally a metal layer is applied to collect the charge carriers. This process is called superstrate configuration.
In producing the solar cells the substrate (preferably glass) forms the base on which the subsequent layers are deposited one after another.
In CdTe solar module preparation normally the thickness of the CdTe layer is maintained in the range of 4 to 5 μm. However, theoretical simulations of the CdTe solar cells show that solar cells with 1 μm CdTe layer could also yield reasonable high efficiency. In principle reducing the CdTe film thickness from 4 to 2 μm could help to reduce CdTe material consumption by 30-40% in module production. The CdTe film thickness reduction would also help to reduce layer deposition time and thereby expedite module production.
High efficiency solar cells are normally achieved with CdTe deposition at substrate temperatures>500° C. The CdTe layer at this temperature has large grains which could result in formation of pinholes. Therefore, simply reducing the layer thickness has several negative influences over the solar cell efficiency and longtime stability. While reducing the film thickness (<3 μm), pinholes are formed in the CdTe layer leading to shunting of the solar cells. This problem will be more pronounced if there is an etching process involved in solar cell production which will lead to poor performance of the solar cell. Furthermore, the reduction in shunt resistance value leads to a low fill factor and eventually reduced efficiency. Therefore, minimizing the pinhole formation in CdTe layer is necessary in order to obtain high efficiency solar cells.
In addition to this, increasing the p-doping of the CdTe layer is also important to achieve high efficiency solar cells. Further increase in efficiency of the CdTe solar cells may be achieved by doping the CdTe layer. According to the theoretical predictions, heavy p-doping of CdTe is limited due to the formation of the self-compensation effect. Only a certain level of p-doping can be achieved by using an appropriate doping element and a process providing a doping element to the CdTe layer after depositing the CdTe layer. During the preparation of the CdTe solar cell, the extrinsic p-doping of CdTe layer is normally done after the activation process and involves post annealing treatment to induce diffusion of doping elements. The well-known and easy p-dopant for CdTe layer is Cu.
The object of the present invention is to obtain a solar cell comprising a doped CdTe layer with a reduced thickness and without pinholes. Furthermore, it is the object of the present invention to simplify the production process of CdTe solar cells.